MONGOID-5222 fix tests

Author: neilshweky

lib/mongoid/extensions/big_decimal.rb

@@ -72,6 +72,8 @@ def mongoize(object)
                 object
               elsif object.is_a?(BigDecimal)
                 BSON::Decimal128.new(object)
+              elsif object.numeric?
+                BSON::Decimal128.new(object.to_s)
               elsif object.respond_to?(:to_d)
                 BSON::Decimal128.new(object.to_d)
               end

spec/mongoid/mongoizable_spec.rb

@@ -81,7 +81,7 @@
   end
 
   describe BigDecimal do
-    let(:invalid_value) { "invalid_value" }
+    let(:invalid_value) { [] }
     let(:klass) { described_class }
     let(:field_name) { :big_decimal_field }